1. Technical Field
One or more of the embodiments of the present invention relate to an emission treatment system for removal of regulated combustion components from the exhaust of a combustion process, such as an internal combustion engine.
2. Background Art
Exhaust from a combustion engine typically contains a variety of combustion components or gases such as unburned hydrocarbon (HC), carbon monoxide (CO), particulate matter (PM), nitric oxide (NO), and nitrogen dioxide (NO2), with NO and NO2, collectively referred to as nitrogen oxide or NOx.
Exhaust emission control or treatment systems have used Selective Catalytic Reduction (SCR) catalysts with NH3 for NOx control on engines, such as lean-burn gasoline engines and diesel engines. SCR catalysts use base metals to promote the reaction between NOx and NH3 to produce N2 under lean conditions. Moreover, exhaust emission control systems have employed oxidation catalysts, such as a diesel oxidation catalyst (DOC), to reduce the amount of HC and CO in the exhaust gas.
Base metal/zeolite SCR catalysts are among the leading candidates for treatment of NOx emissions for North American diesel applications using urea as a reducing agent. It is known that these SCR catalysts are sensitive to sulfur poisoning, especially the copper-zeolite formulations. Even with the use of ultra low sulfur fuel, sulfur poisoning is still a durability issue that impacts on SCR catalyst activity.
Sulfur is usually present as SO2 in the feed gas. Insofar as diesel oxidation catalysts (DOCs) will be employed upstream of the SCR catalysts for most applications, it is likely that a portion of the SO2 will be oxidized into SO3. Recently, it was observed that SCR activity was significantly reduced for Cu/zeolite SCR catalysts poisoned by SO3 vs. those poisoned by SO2 as shown in FIG. 1.
There is thus a continuing need to provide an emission treatment system that provides a relatively high NOx reduction efficiency while minimizing system complexity and substantially avoiding the effects of SO3 aging. Such a system can reduce SO3 generation and thus minimize catalyst deactivation by SO3 aging.